1. Field of the Invention
The present invention relates to a variable-shape reflection mirror, in particular, a small-sized variable-shape reflection mirror capable of high-precision shape control, and to a method of manufacturing the variable-shape reflection mirror using semiconductor fabrication technology.
2. Description of the Related Art
In the field of micro-optical systems applied to microoptics, such as optical pickups, a very small variable-focus mirror capable of varying the curvature of its reflective surface has been proposed for the purpose of simplifying a mechanism relating to focusing, etc., which conventionally uses an electromagnetic actuator. The application of such a variable-focus mirror contributes greatly to further miniaturization of small-sized imaging optical systems.
As regards this type of variable-focus mirror, high-precision products can be manufactured at low cost by applying so-called micro-electromechanical system (MEMS) technology. An example of this technology is proposed in Jpn. Pat. Appln. KOKAI Publication No. 2-101402, for instance. The technique of this document is described below.
As is shown in FIG. 1A and FIG. 1B, a fixed-side electrode layer 12 formed of an electrically conductive film is provided on an upper surface of an insulating substrate 11 formed of, e.g. glass. A silicon dioxide (SiO2) film 14 is formed as an insulating film on one major surface of a silicon substrate 13. A recess 15 is formed on a central portion of the other major surface of the silicon substrate 13. The recess 15 enables a central portion of the SiO2 film 14 to be displaced in its thickness direction. In addition, a movable-side electrode layer 16 is laminated on the SiO2 film 14. Central portions of the SiO2 film 14 and the electrode layer 16 constitute a mirror portion 17. With a voltage applied between the electrode layers 12 and 16, the mirror portion 17 is deformed in a convex shape toward the fixed-side electrode layer 12.
The silicon substrate 13 is coupled to the insulating substrate 11 via a spacer 18, with the SiO2 film 14 being situated downward (in FIGS. 1A and 1B). Further, an SiO2 film 19 is formed on the other major surface of the silicon substrate 13.
A method of manufacturing the above-described mirror device will now be explained with reference to FIGS. 2A to 2E. To start with, as shown in FIG. 2A, SiO2 films 14 and 19 each having a thickness of 400 nm to 500 nm are formed on both mirror-polished surfaces of a silicon substrate 13, which has a plane direction <100>. A metal film with a thickness of about 100 nm is formed as an electrode layer 16 on the lower-side film 14. Then, as shown in FIG. 2B, a photoresist 20 with a predetermined pattern is coated, and a circular window 21 is formed by photolithography. Using the photoresist 20 as a mask, an opening is formed in the SiO2 film 14 with a hydrofluoric-acid-based solution, with the lower-side surface of the substrate being protected. Subsequently, as shown in FIG. 2C, the silicon substrate 13 is immersed in an aqueous solution of ethylenediamine Pyrocatechol and the silicon substrate 13 is etched from the window 21 shown in FIG. 2B. The etching stops when the SiO2 film 14 on the lower side of the substrate 13 is exposed. As a result, a film mirror portion 17 formed of the SiO2 film 14 and electrode layer 16 remains.
On the other hand, as shown in FIG. 2D, a metal film with a thickness of 100 nm, which serves as a fixed electrode, is formed as an electrode layer 12 on the upper surface of the insulating substrate 11 having a thickness of 300 μm. As is shown in FIG. 2E, the silicon substrate 13 is bonded to the insulating substrate 11 with a polyethylene spacer portion 18 with a thickness of about 100 μm interposed, whereby the mirror device shown in FIGS. 1A and 1B is manufactured.
In the above-described variable-shape mirror, a uniform potential difference is provided between the SiO2 film 14 and the fixed-side electrode layer 12. The deformation shape in this case is generally as shown in FIG. 3, compared to a spherical surface having an equal maximum deformation amount. In particular, the amount of deformation in a peripheral portion is deficient and a large spherical aberration occurs. Consequently, high focusing performance cannot be attained. Moreover, when a small-sized mirror is applied to an imaging optical system, oblique light incidence occurs in usual cases. In such cases, in order to obtain good focusing performance, a rotation-asymmetric aspherical surface is required.
To meet this requirement and to deform the variable-shape mirror in a desired shape or an ideal shape, there is an idea of the fixed-side electrode layer being divided into a plurality of regions and different potential differences provided between the divided regions, on the one hand, and the electrode of the deformable surface, on the other hand. Examples of the division mode of the electrode include a concentric shape, a lattice shape and a honeycomb shape. For instance, J. Opt. Soc. Am., Vol. 67, No. 3, March 1977, “The membrane mirror as an adaptive optical element”, proposes a method of dividing the fixed-side electrode in a honeycomb shape.
In addition, the paper of the Japan Society for Precision Engineering, Vol. 61, No. 5, 1995, entitled “Aberration reduction of Si diaphragm dynamic focusing mirror”, discloses a method for making the shape of deformation conform to a specific shape such as a spherical surface shape or a parabolic surface shape. In this method, a deformable surface having a different thickness from location to location is formed.